A joint U.S. Environmental Protection Agency/AOAC interlaboratory method validation study was conducted on EPA Method 507, Determination of Nitrogen- and Phosphorus-Containing Pesticides in Finished Drinking Water by Gas Chromatography with a Nitrogen-Phosphorus Detector, to determine the mean recovery and precision for analyses of 45 nitrogen- or phosphorus-containing pesticides in reagent water and finished drinking waters. The study design was based on Youden's nonreplicate plan for collaborative tests of analytical methods. The waters were spiked with 45 nitrogen- or phosphorus-containing pesticides at 6 concentration levels, prepared as 3 Youden pairs. Ten volunteer laboratories extracted the spiked test waters with methylene chloride, performed a solvent exchange with methyl tert-butyl ether, and analyzed an aliquot of each extract by gas chromatography using a nitrogen-phosphorus detector. Results were analyzed using an EPA computer program, which measured recovery and precision for each of the 45 pesticides and compared the performance of the method between water types. Method 507 was judged acceptable for all analytes tested except merphos, which thermally decomposed in the injection port of the gas chromatograph. Five compounds (carboxin, disulfoton, metolachlor, pronamide, and simazine) exhibited statistically significant matrix effects for the finished drinking water. The method has been adopted official first action by AOAC.